The capacitance of a capacitive sensor changes when an object approaches or touches the sensor. Since the sensors require no moving parts, capacitive sensors may be robust and reliable and widely used in many areas. In particular, capacitive sensors are used in human-to-machine interfaces such as buttons, jog wheels, switches, scroll bars and touch screens.
In many applications, capacitive sensors interface to digital electronic controllers via a capacitance-to-digital converter. Sigma-delta capacitance-to-digital converters have been used successfully in many applications. In a sigma-delta converter, a sigma delta modulator generates a binary sequence of zeros and ones that indicate whether the charge accumulated by the capacitance of the sensor is greater than or less than a reference charge accumulated on a reference capacitor. The sequence of zeros and ones may be integrated and decimated to determine the relationship of the sensor's capacitance to the reference capacitance.
One limitation of this approach is that the reference capacitance must be greater than the sensor capacitance. However, if the capacitance is too large, the sensitivity of the converter is reduced. One approach to reduce this limitation is to adjust the sampling time of the reference capacitance relative to the sampling time of the sensor capacitance. Another approach is to a use an additional offset capacitor that is clocked out of phase with the excitation signal. A still further approach is to adjust the voltage of the excitation signal.
In practice, the impedance of the sensor is not purely capacitive. Hence, a further limitation is that the conversion is that the conversion speed is limited by the discharge time of the sensor capacitance. The discharge time increases as the resistive component of the sensor impedance increases. This can be a significant limitation for applications such as touch screens, which utilize a matrix of sensing elements and require multiple conversions for a single position estimate.
A further limitation is that electromagnetic interference generated by the converters is concentrated in very narrow frequency bands that are multiples of the clock frequencies.
A still further limitation is that a converter may be sensitive to noise, such as electromagnetic interference from synchronous components.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.